Anti-lock braking systems (ABS) typically include some means for sensing wheel speed. ABS systems for air-braked trucks generally include an electronic control module (ECM), a pneumatic control module (PCM) and one or more wheel speed sensors. The wheel speed sensors provide wheel speed information to the ECM and the ECM signals the PCM to modify air pressure level at the brake chambers. This whole process controls the braking level so that the wheels continue to rotate, or at least rotate most of the time, even during heavy braking. The terms hydraulic control module (HCM) or hydraulic control unit (HCU) are more commonly used for larger vehicles with hydraulic brakes.
The wheel speed input is usually in the form of electrical signals indicative of the angular velocity of the wheel. These signals are generally derived from a stationary variable reluctance (VR) sensor coupled to the axle and a toothed or notched “tone wheel” which rotates with the wheel relative the magnet. As the tone wheel passes the sensor, the resulting variations in magnetic flux produce a signal, the frequency of which is a function of the angular velocity of the wheel.
Typically, the VR sensor is mounted on the axle and a sensor coupling wire connects the electromagnetic sensor with the ABS or, more specifically, the ECM of the ABS. For heavy duty vehicles with solid axles, the connecting wire between the sensor and ECM must be disposed exterior to the axle and spindle. Therefore, the addition of an ABS system to these vehicles is relatively expensive because a costly modification to the wheel/axle assembly is required
The stationary magnetic sensor is mounted to the end of the spindle. The tone wheel or magnetic ring is mounted within the wheel cover or hub cap rather than within the wheel housing so that only the cover needs be removed to allow access to the tone wheel and sensor. However, placing the tone wheel immediately inside the wheel cover leaves the tone wheel vulnerable to accidental damage when the wheel cover is removed during normal axle maintenance. Further, the accuracy of the sensor depends on maintaining the gap between the sensor and the tone wheel throughout wheel rotation. Maintaining the sensor-tone wheel gap increases the cost of manufacturing and maintenance costs associated with this type of wheel speed sensing system.
For example, bearing adjustment of the wheel determines how precisely the wheel hub is constrained in an axial direction. As a result, if the bearing adjustment varies, the wheel hub can move axially in service thereby affecting the spacing or gap between the tone wheel and the electromagnetic sensor. Variances and the spacing between the tone wheel and electromagnetic sensor can result in ECM not operating optimally thereby affecting the performance of the ABS. If the spacing discrepancy becomes severe, the ECM may determine that the signal is unusable and ABS may shut down.
Another issue with the prior art sensing systems is that the signal is sensitive to any wobble or “run-out” of the tone wheel and/or the wheel hub. If the plane of the face of the tone wheel is not perpendicular to the axis of rotation, the gap between the tone wheel face and the electromagnetic sensor varies as the tone wheel rotates. This variation causes effects similar to the loose bearing adjustment noted above.
Therefore, in general, wheel speed sensors for ABS systems require a relative rotation between one element such as a toothed ring or tone wheel and a stationary element in the form of an electromagnetic sensor. Coupling wires are needed to carry the signal from the electromagnetic sensor back to the ECM. Because coupling wires are needed between the sensor and the ECM, in practice, the electromagnetic sensor is stationary and the toothed ring or tone wheel rotates. This basic design is not only expensive to install, particularly if the wires from the sensor to the ECM run outside the axle, it is also relatively expensive to maintain and can be prone to inaccurate measurements between maintenance sessions. A less expensive and more reliable system is needed.